1. Field of the Invention
The present invention generally relates to the control of AC induction motor drives requiring a constant-power operating range, such as a battery-powered vehicle drive. More particularly, the invention relates to a motor torque and flux controller usable with such a drive.
2. Description of the Prior Art
The development of practical battery-powered vehicles for widespread use has presented a number of substantial technical challenges. Specifically, the realization of desired operating characteristics is greatly complicated by the fact that these vehicles are powered by on-board storage batteries. This is because the voltage available from the batteries varies significantly depending on state of charge. Also, storage batteries are known to have appreciable source resistance. Thus, the available voltage is also dependent on the load.
A battery-powered vehicle is an example of a drive application requiring a constant power operating range. Such applications must have a constant maximum torque capability up to some "base speed." Above base speed, the maximum torque capability drops off inversely as speed is increased. In the motor, this corresponds to operating with full rated flux up to the base speed, at which point the terminal voltage reaches its desired maximum value. Above base speed, the motor flux must be reduced so that the terminal voltage does not rise any further. Assuming a constant maximum current capability, this insures that approximately constant power is supplied to the motor above base speed.
Constant-power applications have been implemented in modern drive systems utilizing a technique known as "vector control." In these systems, actual motor conditions are typically compared with instantaneous demand signals. Error signals thus produced are processed and converted to polyphase currents needed by the motor to produce a desired torque and magnetic flux density. Vector control theoretically offers the possibility of separately controlling the torque and flux with no cross coupling. The ideal form of this control can be realized provided that some means exist to inject the desired currents into the stator winding, and also provided that the rotor circuit time constant (T.sub.2) is precisely known. In practice, these conditions are never exactly met.
In the case of a battery-powered vehicle, it is even more difficult to provide the constant-power range. The objective in this case is to accelerate the vehicle as rapidly as possible under whatever battery condition prevails. Since the available voltage varies so widely, a constant flux versus speed characteristic has been found to be impractical.